IRS suspension

[GS guy]

G - seems like you're confusing suspension pivot points with drive axles.
A swingaxle drive axle only has 1 pivot point - at the differential. The suspension pivots use that as one of the pivot points, and the 2nd at the torsion bar - as you described.
While the swingaxle and IRS designs share similar numbers of suspension pivot points (at 2 as you stated), the IRS semi-trailing arm has the advantage of Significantly better camber control through the full suspension travel - at the expense of added weight and complexity. It also provides a controlled amount of camber gain in roll, along with a lower roll center. The reason it can't be used in a car like the Spyder is you'd have to locate the inner forward pivot point inside the engine! You could, however, adapt a 914 trailing arm design and use the IRS style trans - again at the expense of added weight and complexity.

There's a reason all auto manufacturers got away from using the swingaxle suspension (with some rare exceptions - like the Ford twin I-beam pickup front suspension) - roll center is too high and too difficult to control the huge camber changes while maintaining decent suspension travel. It certainly is less expensive and much simpler to build from a manufacturers cost perspective!
However, in certain specific applications (smooth pavement and very limited suspension travel) it can be made to work well - especially with added features like using a Z bar or ZRS design.
Jeff

Color me ignorant, but what is ZRS?

[GS guy]

Originally Posted by vwdevotee

Color me ignorant, but what is ZRS?

It's a swingaxle based suspension that uses zero roll resistance at the rear, but makes the swinger behave much like a solid axle car. It allows the rear-chassis to roll without resistance - but at the same time keeps both tires planted flat on the ground - regardless of roll angle. All vehicle roll resistance is controlled up front, with a standard sway bar. SOP on current formula Vee designs, though it was created back in the late 60's (or maybe before).

Since the roll resistance is all controlled by the front tires and sway bar, doesn't that put a huge strain on the outer front tire? Doesn't a ZRS car push bad in the corners?

[GS guy]

Originally Posted by vwdevotee

Since the roll resistance is all controlled by the front tires and sway bar, doesn't that put a huge strain on the outer front tire? Doesn't a ZRS car push bad in the corners?

You'd think it would. I don't know anything about the dynamics of driving a ZRS equipped car, only the concept of how it works and intent. Apparently it works very well since it's been used for over 20 years in Formula Vee. The modern version also uses a different trailing arm (leading arm?) axle positioner that minimizes toe changes in bump/rebound.

I've noticed though that a lot of archaic technologies are still used in racing series, but they are typically there because of some rule. I would say that, while the ZRS setup might keep the tires at a constant angle to the ground, since Porsche abandoned it very early on for the sake of performance, there are much better geometries.

[GS guy]

Originally Posted by vwdevotee

I've noticed though that a lot of archaic technologies are still used in racing series, but they are typically there because of some rule. I would say that, while the ZRS setup might keep the tires at a constant angle to the ground, since Porsche abandoned it very early on for the sake of performance, there are much better geometries.

Agreed! The unequal length A-arm suspension rules - it's a winning design and all the highest level performance cars utilize it (when they could do anything they want, with virtually unlimited budgets!). Only the computer controlled "active" suspension of the F1 cars from the 90's (was it the 90's?) surpasses the tried and true classis a-arm coil-over design - and it was still a version of the A-arm layout. But dealing with the classic VW Beetle, you're somewhat limited by what advancements you can employ. Utilizing applicable Porsche components seems to the "ultimate" at this point. Unless you want to totally re-engineer the chassis.... Pretty much exactly what I'm doing with my Deserter buggy.

[G-force]

Originally Posted by GS guy

G - seems like you're confusing suspension pivot points with drive axles.

No, sorry bud, I'm not confusing anything. Thanks for the 'lecture' though, it was interesting...

The swingaxle has exactly the same number of joints as the IRS semitrailing-arm suspension, and I've never said otherwise.

Likewise the number joints in the transmission has nothing to do with the suspension geometry. A simple fact which most people on here seem to have missed, yourself included.

Originally Posted by GS guy

The IRS semi-trailing arm has the advantage of Significantly better camber control through the full suspension travel - at the expense of added weight and complexity.

Not true at all. It just reduces camber changes to 1/3 the value of a swingaxle when travelling in a straight line. This is good for stability but doesn't improve cornering ability at all.

Do some research, you'll find the semi-trailing arm design doesn't make the car corner better, just gives a more linear response and therefore a more predictable ride and better straight line stability that's all.

The IRS car will have more body roll and likewise more positive camber than the swingaxle in the same corner, since the suspension on a swingaxle car is not affected AT ALL by body roll. That's a simple fact of the laws of physics that most beetle 'experts' really don't seem to realise.

In fact body roll causes positive camber on the IRS semi-trailing arm design. And some negative camber on the swingaxle.

BOTH cars suffer from jacking GS_guy, and this also causes more positive camber in the corner. The semi-trailing design just suffers 1/3 the amount of jacking that the swingaxle gets.

So at the end of the day if you work out the math you find they generally have the same amount of positive camber in the same corner, and neither car has more grip.